Optical data storage medium and use of such medium

ABSTRACT

An optical data storage medium ( 10 ) for recording by means of a focused radiation beam ( 9 ) having a wavelength λ is described. The beam enters through an entrance face ( 8 ) of the medium during recording. The medium at least comprises a substrate ( 1 ), including a guide groove with a depth g. The guide groove is present at the side of the substrate opposite to the entrance face. A recording stack ( 2, 3 ) of layers is present adjacent the substrate ( 1 ) at the side of the guide groove. The stack includes a write once recording layer ( 2 ) of a material having a complex refractive index ñ R =n R −i*k R  at the wavelength λ and having a thickness d RG  in the groove portion and a thickness d RL  in the portion between grooves. A non-metallic layer ( 3 ) of a substantially transparent material, is present adjacent the write-once recording layer ( 2 ). The groove depth g is in the range (λ/655)*20 nm&lt;g&lt;(λ/655)*140 nm with λ expressed in nm. This range achieves a sufficient push-pull tracking signal and a sufficient modulation of recorded marks.

The invention relates to an optical data storage medium for recording bymeans of a focused radiation beam having a wavelength λ and enteringthrough an entrance face of the medium during recording, at leastcomprising:

-   -   a substrate, including a guide groove with a depth g, the guide        groove being present at the side of the substrate opposite to        the entrance face,    -   a recording stack of layers on the substrate at the side of the        guide groove, which stack includes:    -   a write once recording layer of a material having a complex        refractive index ñ_(R)=n_(R)−i*k_(R) at the wavelength λ and        having a thickness d_(RG) in the groove portion and a thickness        d_(RL) in the portion between grooves, being present adjacent        the substrate,    -   a non-metallic layer of a substantially transparent material,        being present adjacent the write-once recording layer.

The invention also relates to the use of such an optical data storagemedium in a standard optical data storage medium reading/recordingdevice.

One of the driving factors in the optical data storage field is theincrement of the data capacity. At present a dual stack DigitalVersatile Disk Recordable medium (DL-DVD+R) is being developed, whichwill increase data storage capacity by almost a factor two on a 12 cmDVD recordable disk: 8.5 GB on dual-layer DVD+R compared to 4.7 GB on asingle layer DVD+R. A further doubling of data storage capacity can begained by moving to quadruple-stack DVD recordable disks (QL-DVD+R).Most likely, such a quadruple-stack medium will also be based onreflective storage layers. Switchable layers, e.g. thermochromic,photochromic, or electrochromic, are less likely to be considered atpresent. Note that the term stack is often referred to as layer,although a stack comprises two or more layers. The terms medium and diskare used interchangably.

In case of dual-stack DVD+R disk it has been recognized that dyes arethe most attractive candidates as recording material due to theirintrinsic high transparency at the recording/reading wavelength.Therefore, also for multi-stack disks dyes will be used as the recordingmaterial. Most likely, for the deepest stack a conventional DVD+R stackdesign can be used. Multi-stack designs may be represented by a symbolLn in which n denotes 0 or a positive integer number. The stack at whichthe radiation beam arrives first, i.e. the stack closest to the entranceface, is called L0, while each stack further from the radiation sourceis represented by L1 . . . Ln. Thus in case of dual stack media twostacks L0 and L1 are present in which design L0 denotes the “top”recording layer and L1 denotes the “deepest” recording layer. The L0stack in a dual-stack DVD+R may use a thin semi transparent metallicreflective layer, e.g. a 10 nm Ag layer. Such an L0 stack has atransmission of about 60%. However, in QL-DVD+R disks, further L2 and L3stacks are present and the L0 and L1 stacks require even highertransmission values of 70-80% in order to achieve sufficient signal fromthe deeper L2 and L3 stacks. Increasing the transmission by using eventhinner metallic layers is not an option because layer homogeneitybecomes problematic. However, high-transparency stacks can be obtainedby combining dyes with non-metallic reflective layers, e.g. dielectricmirrors, which are known in the art.

For truly useful stack designs, several parameters must besimultaneously optimized: reflection and transmission, modulation ofwritten marks and servo tracking signal for each of the stacks.

To be able to track an empty recordable optical disk (eithersingle-stack, dual-stack, or multi-stack), so-called guide grooves orpre-grooves are present in the substrate or intermediate layer on whichthe optical recording stack is deposited. The pre-grooves result in aphase-difference between light reflected from the grooves and lightreflected from the portion in between the grooves (lands). As aconsequence of the different complex reflection amplitudes on land andgroove, the incoming radiation beam, e.g. laser-light, is diffracted.When detected properly, the interference between the ±1st and 0thdiffracted orders of the reflected light results in the so-calledpush-pull signal which can be used by an optical tracking system to keepthe laser-light spot on the pre-grooves. In practice this method employstwo radiation-sensitive detectors arranged in the path of the beam thathas been reflected from the optical data storage medium so that thedetectors receive radially different portions of the reflected beam. Thedifference between the output signals of the two detectors containsinformation about the radial position of the laser spot relative to thegroove. If the output signals are equal, the center of the laser spotcoincides with the center of the groove or the center between twoadjacent grooves. Hence during recording the groove is employed fordetecting the radial position of the laser light write spot formed onthe recording layer by the focused laser beam, relative to a groove, sothat the radial position of the write spot can be corrected. As a resultof this, less stringent requirements have to be imposed on the drive andguide mechanism for moving the write beam and the optical data storagemedium relative to each other, enabling a simpler and cheaperconstruction to be used for the write apparatus. In order for an opticaldrive to track properly on an empty disk, it is essential that thepush-pull signal has both the correct sign and a sufficient value. Therequired values are usually specified in the standard of the specificoptical disk. In general, both the sign and amplitude of the push-pullsignal are to a large extent governed by the phase difference betweenlight reflected from land and groove. Usually the guide groove orpregroove track comprises a spiral groove in the transparent substrateor intermediate layer and the recording layer is a thin layer of, forexample, an organic dye. The guide groove extends across the entireoptical data storage medium surface. The focused laser light beam, ofsufficiently high intensity can produce an optically detectable changeor mark in the recording layer. The modulation depth M of such writtenmarks is defined as the difference in the light intensity received froman unwritten part of the groove and the light intensity from a writtenpart of the groove normalized to the maximum of the two intensities.

It has been found that layers of specific dyes are very suitable for useas a recording layer on a pre-grooved optical data storage mediumsubstrate. Such a dye may, for example, be a cyanine dye or an azo dye,which can be deposited by spincoating a solution of such a dye on thesubstrate surface. When a layer of dye is applied to a pre-groovedoptical data storage medium substrate the grooves are filled partiallyor completely and the thickness of the layer at the location of thegrooves d_(RG) will generally be larger than the thickness d_(RL)between the grooves. The area between the grooves is also calledon-land. As a result of this difference in layer thickness, which isequal to the d_(RG)−d_(RL), an additional phase shift occurs between theradiation reflected from the recording layer at the location of a grooveand radiation reflected from the recording layer at the location of aland. This additional phase shift gives rise to a differential trackingsignal which is different from the case in which d_(RG)=d_(RL). Aleveling parameter may defined as: L=(d_(RG)−d_(RL))/g. When L=1 thegrooves are completely flattened out by the recording layer, that is thegroove structure is not present anymore in the surface of the recordinglayer opposing the substrate. This may occur for very shallow grooves(g<<d_(RG)). However, in most practical cases, e.g. Compact DiskRecordable (CD-R) or DVD Recordable (DVD+R) disks, the levelingparameter L ranges from 0.2 to 0.5. For instance, for a typical DVD+R,the groove depth is 160 nm, the dye thickness in the groove is 100 nmand the dye thickness on-land is 40 nm: L=(100−40)/160=0.375. When thedye is deposited by a different technique such as evaporation theleveling can be nearly zero, i.e. the same thickness of dye on-land andin-groove.

It is an object of the present invention to provide an optical datastorage medium of the kind described in the opening paragraph, which hasa sufficient push-pull signal and a sufficient modulation of recordedmarks.

This object is achieved in accordance with the invention by an opticaldata storage medium as described in the opening paragraph, which ischaracterized in that the groove depth g is in the range (λ/655)*20nm<g<(λ/655)*140 nm with λ expressed in nm.

The invention is based on the recognition of the problem that for anoptical storage medium according to the opening paragraph having anon-metallic reflective layer the value of the push-pull signal of thegroove and the value of the mark modulation are not sufficient. As shownin FIG. 3 there is a substantial difference between the normalizedpush-pull signal PP (defined below) in case of a metallic and anon-metallic reflective layer. Even more important, for the typicalgroove depth of 170 nm used in single-layer DVD+R with metallicreflective layer, the push-pull in the case of dye-on-dielectric stackis nearly zero, which implies that tracking on such a disc ispractically impossible. The guide groove, normally formed as a spiral,has a pitch p and preferably has an average width w in the range of 0.3to 0.7 times p. For DVD the pitch p is approximately 0.74 μm. For DVDthe wavelength λ is approximately 655 nm. For different wavelengths theoptimum range needs to be scaled accordingly, e.g. for λ=405 nm multiplyby 405/655. Hence the optimum range for λ=405 nm would be (405/655)*20nm<g<(405/655)*140 nm. Generally the push-pull signal is derived bysubtracting the signals I_(R) and I_(L) from the right and left detectorhalve of a split detector that is present in the reflected light path ofthe laser beam during scanning of the guide groove. In optical diskstandard specifications the push-pull signal is normally defined as anormalized parameter PP=<I_(R)−I_(L)>/[I_(R)+I_(L)] in which formula<I_(R)−I_(L)> denotes the maximum difference of I_(R)−I_(L) and[I_(R)+I_(L)] denotes the average value of I_(R)+I_(L) when the laserspot moves radially outwards across the guide grooves. Note that this PPis not the same as the unnormalized push pull signal denoted by PP (initalics) which can be defined as (I_(R)−I_(L)). The shape of the graphof the normalized push-pull signal PP for a stack, including anon-metallic reflective layer, as a function of groove depth isconsiderably different from the case with a normal metallic reflectivelayer, which is shown in FIG. 3. A different track pitch and/or groovewidth may slightly influence the amplitude of the push-pull, but thiseffect is considerably smaller than the effect of groove depth. Normallythe groove is shaped as shown in FIG. 1 in which drawing the definitionof the groove depth is shown. According to the DVD+R standard, the phasedepth of the grooves should not exceed 90 degrees, this means that inthe presented calculations the push-pull of the normal stack should bepositive.

The recognized problem outlined above can be solved by using the claimedrange of groove depths in case of a non-metallic reflective layercompared to normal range of groove depths 150 nm to 180 nm forconventional disks having a metallic reflective layer. The advantage ofthis solution is that radial push-pull tracking on such a disk having astack with a non-metallic reflective layer becomes possible and thatfurthermore the modulation of written marks is sufficient.

In an embodiment the non-metallic layer mainly comprises a materialselected from the group of transparent plastic, silicon, oxides ofsilicon, nitrides of silicon and carbides of silicon.

These materials are suitable candidates because they have a relativelyhigh transparency and are relatively stable. Other suitable dielectricmaterials are ZnS—SiO₂, and oxides and nitrides in general.

For λ=655 nm, e.g. used for DVD, it is preferred that 20 nm<g<125 nm. Itis important for reliable readout that the modulation is maximized. Inthe groove depth range g>125 nm the modulation M drops to relativelysmall values. Therefore the said range of groove depth g for anon-metallic reflective layer recordable DVD-type stack is preferred.

For λ=655 nm, it is preferred that 50 nm<g<125 nm because for veryshallow grooves the push-pull signal PP may become relatively too smallwhich will result in unreliable tracking.

In an embodiment, in which λ=655 nm, the recording layer has a thicknessd_(RG) and 145 nm≦d_(RG)*n_(R)<245 nm and the non-metallic layer mainlycomprises SiO₂ and has a thickness d_(T) in the range 10 nm≦d_(T)≦120nm. In the preferred embodiment with this non-metallic layer materialthe following approximate values apply: d_(T)=110 nm, d_(RG)=80 nm, g=80nm, the dye is an azo dye with ñ_(R)=2.45−i*0.08 at the recordingwavelength.

In another embodiment, in which λ=655 nm, the recording layer has athickness d_(RG) and 132 nm≦d_(RG)*n_(R)<220 nm and the non-metalliclayer mainly comprises SiC and has a thickness d_(T) in the range 10nm≦d_(T)≦60 nm. In the preferred embodiment with this non-metallic layermaterial the following approximate values apply: d_(T)=52 nm, d_(RG)=70nm, g=120 nm, the dye is an azo dye with ñ_(R)=2.24−i*0.02 at therecording wavelength.

In a further embodiment, in which λ=655 nm, the recording layer has athickness d_(RG) and 154 nm≦d_(RG)*n_(R)<264 nm and the non-metalliclayer mainly comprises amorphous Si (a-Si) and has a thickness d_(T) inthe range 1 nm≦d_(T)≦20 nm. In the preferred embodiment with thisnon-metallic layer material the following approximate values apply:d_(T)=10 nm, d_(RG)=100 nm, g=120 nm, the dye is an azo dye withñ_(R)=2.24−i*0.02 at the recording wavelength.

In another embodiment at least one further recording stack is presentadjacent a further substrate, including a guide groove with a depth g.in the same range as g, the guide groove being present at the side ofthe further substrate opposite to the entrance face, the furtherrecording stack including:

-   -   a further write once recording layer of a material having a        complex refractive index ñ′_(R)=n′_(R)−i*k′_(R) at the        wavelength λ and having a thickness d′_(RG) in the groove        portion and a thickness d′_(RL) in the portion between grooves,        being present adjacent the substrate,    -   a further non-metallic layer of a substantially transparent        material, being present adjacent the further write-once        recording layer. The recording stack including the non-metallic        reflective layer may be repeated in order to achieve a multi        stack recordable medium. The use of the non-metallic layer is        advantageous because a relatively high transmission is possible        with a non-metallic reflective layer. Especially when using        three or more recording stacks non-metallic layers are        advantageous because of their relatively high optical        transmission.

The substrate of the optical data storage medium is at least transparentfor the radiation beam wavelength. For DVD the substrate is disk-shapedand has a diameter of 120 mm and a thickness of 0.6 mm and a furthersubstrate with a thickness of 0.6 mm, the recording stack beingsandwiched between the substrate and the further substrate. The guidegroove is often constituted by a spiral-shaped groove and is formed inthe substrate or further substrate by means of a mould during injectionmolding or pressing. These grooves can be alternatively formed in areplication process in a synthetic resin, for example a UV light-curableacrylate, which serves as the further substrate after curing.

Use of the optical data storage medium according to the invention in astandard optical data storage medium recording/reading device suitablefor tracking by means of the push pull method onto a guide groove of astandard recordable optical data storage medium, which guide groove ispresent near a metallic reflective layer, has the advantage that nomodification in the push-pull signal processing electronics of therecording/reading device is required. The push-pull signal will have asufficient value.

The invention will be elucidated in greater detail with reference to theaccompanying drawings, in which

FIG. 1 is a schematic layout of an optical storage medium according tothe invention.

FIG. 2 is a schematic layout of an optical storage medium according tothe invention having two recording stacks.

FIG. 3 shows the normalized push-pull of dye on a metallic (Ag) metallicreflective layer and on a dielectric (SiO₂) reflective layer versusgroove depth g at λ=655 nm.

FIG. 4A shows the normalized push-pull PP for a 80 nm AZO-dye/110 nmSiO2 stack for three values of leveling L as a function of the groovedepth g at λ=655 nm.

FIG. 4B shows the modulation M for a 80 nm AZO-dye/110 nm SiO2 stack forthree values of leveling L as a function of the groove depth g at λ=655nm.

FIG. 5A shows the normalized push-pull PP for a 70 nm AZO-dye/52 nm SiCstack for three values of leveling L as a function of the groove depth gat λ=655 nm.

FIG. 5B shows the modulation M for a 70 nm AZO-dye/52 nm SiC stack forthree values of leveling L as a function of the groove depth g at λ=655nm.

FIG. 6A shows the normalized push-pull PP for a 100 nm AZO-dye/10 nma-Si stack for three values of leveling L as a function of the groovedepth g at λ=655 nm.

FIG. 6B shows the modulation M for a 100 nm AZO-dye/10 nm a-Si stack forthree values of leveling L as a function of the groove depth g at λ=655nm.

In FIG. 1 a schematic cross section of an optical data storage medium10, according to the invention, for recording by means of a focusedradiation beam 9 is shown. The radiation beam is a laser beam and has awavelength λ of approximately 655 nm and enters through an entrance face8 of the medium during recording. The numerical aperture (NA) of thefocused beam is 0.65. The medium comprises a substrate 1, including aguide groove with a depth g. The guide groove is present at the side ofthe substrate opposite to the entrance face 8. A recording stack 2, 3 oflayers is present on the substrate 1 at the side of the guide groove.The recording stack includes a write once recording layer 2 of an azodye having a complex refractive index ñ_(R)=2.45−i*0.08 at thewavelength and having a thickness d_(RG)=80 nm the groove portion and athickness d_(RL)=32 nm in the portion between grooves, which correspondsto a leveling L=0.4. The write once recording layer 2 is presentadjacent the substrate 1. Adjacent the write-once recording layer 2 anon-metallic layer 3 made of SiO₂ is present. The groove depth g=80 nm.A further substrate 4 is present adjacent the SiO₂ layer. The values ofthe normalized push-pull signal PP and the modulation M are 0.96 and0.42 respectively, which values are sufficient for proper tracking andread out.

In FIG. 2 a schematic cross section of another embodiment of an opticaldata storage medium 20 according to the invention is shown. Referencenumerals 1, 2, 3, 4, 8 and 9 denote the items as described with FIG. 1.A further recording stack 2′, 3′ is present adjacent the furthersubstrate 4. The further recording stack 2′, 3′ may contain the samematerials as the recording stack 2, 3.

In FIG. 3 the normalized push-pull signal PP of a dye on a metallic Agreflective layer and on a dielectric SiO₂ reflective layer versus groovedepth g are compared. The dye thickness in groove is 80 nm, levellingL=0.4, and the real part of the dye's refractive index is 2.3, λ=655 nmand NA=0.65. The normalized push-pull PP in case of a metallic or adielectric reflective layer is substantially different. It is even moreimportant that for the typical groove depth of 170 nm, used insingle-layer DVD+R with metallic reflective layer, the normalizedpush-pull in the case of dye-on-dielectric stack is nearly zero andtracking on such a disk is practically impossible. In the followingdescription of FIGS. 4A-6B the used wavelength λ=655 nm and NA=0.65.

In FIG. 4A the normalized push-pull PP for a 80 nm AZO-dye/110 nm SiO₂stack for three values of leveling L as a function of the groove depth gis shown. Note that beyond g=125 nm the normalized push-pull value PPshows a decrease and becomes too low for proper tracking. The same holdsfor small values of g, e.g. <20 nm.

FIG. 4B shows the modulation M for a 80 nm AZO-dye/110 nm SiO₂ stack forthree values of leveling L as a function of the groove depth g. Thepreferred groove depth g for this stack is 80 nm.

FIG. 5A shows the normalized push-pull PP for a 70 nm AZO-dye/52 nm SiCstack for three values of leveling L as a function of the groove depthg. It should noted that PP value stays at an acceptable level untilabout g=180 nm. However the modulation M tends to decrease at lowervalues of g. Hence a trade off is made between PP and M.

FIG. 5B shows the modulation M for a 70 nm AZO-dye/52 nm SiC stack forthree values of leveling L as a function of the groove depth g. Notethat beyond g=125 nm the modulation value shows a decrease and becomestoo low for proper read out. The preferred groove depth g for this stackis 120 nm.

FIG. 6A shows the normalized push-pull PP for a 100 nm AZO-dye/10 nma-Si stack for three values of leveling L as a function of the groovedepth g.

FIG. 6B shows the modulation M for a 100 nm AZO-dye 10 nm a-Si stack forthree values of leveling L as a function of the groove depth g. Notethat beyond g=125 nm the modulation value M shows a decrease and becomestoo low for proper read out. The preferred groove depth g for this stackis 120 nm.

It should be noted that the above-mentioned embodiments illustraterather than limits the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

According to the invention an optical data storage medium for recordingby means of a focused radiation beam having a wavelength λ is described.The beam enters through an entrance face of the medium during recording.The medium at least comprises a substrate, including a guide groove witha depth g. The guide groove is present at the side of the substrateopposite to the entrance face. A recording stack of layers is presentadjacent the substrate at the side of the guide groove. The stackincludes a write once recording layer of a material having a complexrefractive index ñ_(R)=n_(R)−i*k_(R) at the wavelength λ and having athickness d_(RG) in the groove portion and a thickness d_(RL) in theportion between grooves. A non-metallic layer of a substantiallytransparent material, is present adjacent the write-once recordinglayer. The groove depth g is in the range (λ/655)*20 nm<g<(λ/655)*140 nmwith λ expressed in nm. This range achieves a sufficient push-pulltracking signal and a sufficient modulation of recorded marks.

1. An optical data storage medium (10) for recording by means of afocused radiation beam (9) having a wavelength λ and entering through anentrance face (8) of the medium during recording, at least comprising: asubstrate (1), including a guide groove with a depth g, the guide groovebeing present at the side of the substrate opposite to the entrance face(8), a recording stack (2, 3) of layers on the substrate (1) at the sideof the guide groove, which stack includes: a write once recording layer(2) of a material having a complex refractive index ñ_(R)=n_(R)−i*k_(R)at the wavelength λ and having a thickness d_(RG) in the groove portionand a thickness d_(RL) in the portion between grooves, being presentadjacent the substrate, a non-metallic layer (3) of a substantiallytransparent material, being present adjacent the write-once recordinglayer (2), characterized in that the groove depth g is in the range(λ/655)*20 nm<g<(λ/655)*140 nm with λ expressed in nm.
 2. An opticaldata storage medium (10) as claimed in claim 1, wherein the non-metalliclayer (3) mainly comprises a material selected from the group oftransparent plastic, silicon, oxides of silicon, nitrides of silicon andcarbides of silicon.
 3. An optical data storage medium (10) as claimedin claims 1 or 2, wherein the wavelength λ is approximately 655 nm. 4.An optical data storage medium (10) as claimed in claim 3, wherein g<125nm.
 5. An optical data storage medium (10) as claimed in claims 3 or 4,wherein g>50 nm.
 6. An optical data storage medium (10) as claimed inany one of claims 3-5, wherein the recording layer (2) has a thicknessd_(RG) and 145 nm≦d_(RG)*n_(R)<245 nm and the non-metallic layer mainlycomprises SiO₂ and has a thickness d_(T) in the range 5 nm≦d_(T)≦120 nm.7. An optical data storage medium (10) as claimed in any one of claims3-5, wherein the recording layer has a thickness d_(RG) and 132nm≦d_(RG) *n_(R)<220 nm and the non-metallic layer mainly comprises SiCand has a thickness d_(T) in the range 5 nm≦d_(T)≦60 nm.
 8. An opticaldata storage medium (10) as claimed in any one of claims 3-5, whereinthe recording layer has a thickness d_(RG) and 154 nm≦d_(RG)*n_(R)<264nm and the non-metallic layer mainly comprises amorphous Si and has athickness d_(T) in the range 1 nm≦d_(T)≦20 nm.
 9. An optical datastorage medium (20) as claimed in any one of the preceding claims,wherein at least one further recording stack (2′, 3′) is presentadjacent a further substrate (4), including a guide groove with a depthg, in the same range as g, the guide groove being present at the side ofthe further substrate (4) opposite to the entrance face (8), the furtherrecording stack (2′, 3′) including: a further write once recording layer(2′) of a material having a complex refractive indexñ′_(R)=n′_(R)−i*k′_(R) at the wavelength λ and having a thicknessd′_(RG) in the groove portion and a thickness d′_(RL) in the portionbetween grooves, being present adjacent the substrate, a furthernon-metallic layer (3′) of a substantially transparent material, beingpresent adjacent the further write-once recording layer (2′).
 10. Use ofan optical data storage medium (10, 20) as claimed in any one of thepreceding claims, in a standard optical data storage mediumrecording/reading device suitable for tracking by means of the push pullmethod onto a guide groove of a standard recordable optical data storagemedium, which guide groove is present near a metallic reflective layer.